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House (English) sparrows and European starlings are non-native species introduced from Europe and their aggressive seizure of cavity nest sites is the main reason for the rarity of bluebirds today. Starlings nest in many of the natural nest sites but can be excluded from nest boxes by only using 1 1/2 or 1 9/16 inch entrance holes. House sparrows can readily enter bluebird nest boxes and frequently kill bluebirds, destroy their eggs, or drive them from their nests. At no time should they be allowed to successfully nest in bluebird boxes. Doing so will increase the house sparrow population and further reduce the number of the bluebirds.

from: http://www.nabluebirdsociety.org/monitoring.htm

12 years ago. Rating: 2
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